Sub-Floor Ventilation Calculator — Airbrick Count & Spacing

Approved Document C (2004 edition incorporating 2010 and 2013 amendments) para 4.14(b) and NHBC Standards 5.2.10 require ventilation openings of not less than EITHER 1500mm² per metre run of external wall OR 500mm² per m² of floor area — whichever gives the greater opening area. The free area you must provide therefore depends on both the perimeter and the floor area; deep, narrow-frontage plans are often governed by the 500mm²/m² floor-area branch. This calculator takes the greater of the two.

It depends on the free area of the vent you fit, so always check the manufacturer datasheet. A square-hole 215×65mm clay airbrick gives only about 1290mm² of free area, so to meet 1500mm²/m you would need roughly one every 0.86m. A telescopic underfloor vent such as the Timloc 1201 gives about 8000mm² (6500mm² with its airbrick), covering far more wall per unit. Whatever the free area, NHBC 5.2.10 caps timber-floor ventilators at no more than 2m centres, within 450mm of each wall end, with openings on two opposing walls for cross-flow — so 2m is the maximum spacing even where the free-area maths would allow fewer.

A telescopic liner is a plastic sleeve that extends through the cavity wall to connect the external airbrick to the sub-floor void. Without it the airflow would enter the cavity instead of the sub-floor space. The liner telescopes to suit a range of brick courses (and cavity widths around 50-100mm) and maintains the cavity insulation integrity. Pipes carrying ventilating air should be at least 100mm diameter.

Airbricks are commonly blocked by soil build-up against walls, paving raised above the airbrick level, render applied over them, or deliberate blocking to reduce draughts. Blocked airbricks are a major cause of sub-floor dampness, timber decay and musty odours. They should always be cleared or replaced.

Yes. Mechanical sub-floor ventilation fans can be installed as an alternative or supplement to airbricks. They are particularly useful where cross-ventilation is poor (e.g. terraced houses with only front and rear walls). Fans should provide at least 3 air changes per hour in the sub-floor void.

Airbricks must be positioned below the DPC (damp proof course) level but above external ground level. If the ground level is above the airbrick, excavation or a dry area (channel) should be formed to expose them. The airbrick should be at least 75mm above external ground level.